Method and device for active impedance matching

ABSTRACT

The disclosure describes a system and method for dynamically matching a source impedance to a load impedance or the complex conjugate of the load impedance. An embodiment of the present invention is a device for active impedance matching comprising a current driver having an output connected to a load, means for detecting an output voltage from the current driver to the load, means for scaling the detected output voltage by a scaling value, and means for subtracting a value representing the scaled detected output current from an input signal of the voltage driver.

BACKGROUND

[0001] This application is a Divisional Application of U.S. patent application Ser. No. 09/790,257 filed on Feb. 21, 2001, now pending and claims the benefit of the disclosure of U.S. Provisional Patent Application Ser. No. 60/185,656, filed on Feb. 29, 2000, the entire content of which is hereby incorporated by reference herein.

[0002] The invention relates generally to transmission lines and, more particularly, to adjusting the terminating and driving impedance of a transmission line to match the characteristic impedance of the transmission line.

[0003] It is well known to skilled practitioners in the electrical arts that if a source impedance is matched to a complex conjugate of a load impedance, maximum power transfer between the source and the load is achieved. However, it is difficult to match the imaginary part of the complex impedance and half of the power is lost in the matched source impedance when using passive components for impedance matching. Although this is a characteristic of many electrical circuits, it may take on greater significance where transmission lines are considered. With transmission lines, the primary objective is to avoid reflections in the transmission line, so the characteristic impedance is assumed to be resistive.

[0004] Transmission lines, where the transmission line length is large with respect to the wavelength of the lowest transmission frequency, are commonly used for transmission of data between two or more locations. It is well known in the art of transmission lines, and particularly transmission lines for transmitting information at high data rates, that in order to maximize the efficiency of information transfer with minimum loss and dispersion effects, the terminating impedance of a receiver and the driving impedance of a transmitter must match the characteristic impedance Z₀ of the transmission line over the frequency range of interest. That is, it is desirable to maintain a uniform characteristic impedance Z₀ along the length of the signal carrying line. Any mismatch in the characteristic impedance across interconnect interfaces will cause reflection of the signal at the interface, resulting in losses and distortion of the signal in the form of attenuation, echo and cross-talk. Furthermore, multiple reflections from multiple interfaces only compound the deleterious affect on the information-carrying signal. The classical solution to the impedance matching problem involves attempting to match the distributed-parameter impedance of the transmission line with lumped-parameter impedances of resistor, capacitor and inductor circuit elements.

[0005] Wide band communication channels, like ADSL modulation over telephone conventional lines or other wideband modulation schemes, require matching of line impedances that are complex, where amplitude and phase are dependent on frequency. Telephone subscriber loops with bridged taps present impedance variations at the receiver end that are difficult to match using simple circuits. Furthermore, the impedances variations may change from loop to loop, making it impossible to design a matching circuit using generic discrete circuit components. The use of full-duplex techniques, where bi-directional transmission is conducted concurrently only further complicates the difficulty of matching interface impedances to the characteristic impedance of the transmission line.

[0006] There have been a number of different approaches to solving the characteristic impedance matching problem. In the most simple and rudimentary form, fixed resistor elements are connected across the transmission line interfaces to match the interface impedance with the characteristic impedance of the transmission line. More complex impedance matching circuits using combinations of resistor and capacitor elements are often found connected to transmission lines. Impedance matching circuits using passive components may dissipate half of the available power at the transmitter, oftentimes reducing its dynamic range by half. Although power is seldom a major consideration on a standard data transmission line, loss in dynamic range can result in excessive signal clipping with high peak to average ratios that are typical of Quadrature Amplitude Modulated signals and Discrete Multi-Tone signals, used in many modem data transmission systems.

[0007] One of the oldest and widely used approaches to match a transmitter-receiver to a transmission line is a hybrid circuit that makes use of two transformers and a balance impedance network Z_(L) that, when matched to the characteristic impedance Z₀ of the transmission line, results in very high isolation between transmitter and receiver circuits. This circuit provides a line termination that matches the characteristic impedance of the line and results in no reduction in dynamic range. However, only half the power delivered by the transmitter is sent to the transmission line, the other half being wasted on the balancing impedance network Z_(L). In addition to loss of transmitted power, the balancing impedance network Z_(L) cannot perfectly match a line with bridged taps or multiple interfaces. It is impractical to add switching circuits to adapt the impedance to different lines, where each line has a different configuration of taps or interfaces along the length of the line. Furthermore, this hybrid circuit makes use of multiple magnetic circuits that have inherent non-linear characteristics that produce distortion, which adversely affects signals with high peak to average ratios. These transformers also exhibit parasitic capacitance and leakage inductance that may impair circuit operation and reduce useful bandwidth.

[0008] Another approach that has received increased interest is the use of a differential driver circuit having two outputs, where each output is connected through an impedance matching resistor to each of the two terminals, respectively, of the primary winding of a transformer. The secondary winding of the transformer is connected to the transmission line. However, not only is half of the transmitter power dissipated in the two impedance matching resistors, but half of the signal amplitude is also dropped across these resistors. This results in reducing the dynamic range of the signal at the transmitter by one-half and reducing the maximum power available to drive the transmission line by one-fourth. The transformer provides for scaling the line impedance to compensate for this reduction and for generating enough peak voltage without excessive clipping. Two amplifiers, each connected across a terminating resistor receive the signal on the transmission line. This circuit may only perform better than the hybrid circuit described above in the high frequency range, where the line impedance will be mostly resistive in nature. Although more complex networks may replace these terminating resistors, the resultant configuration would also suffer from the same limitations as the hybrid circuit described above, namely low power efficiency and reduced dynamic range.

[0009] All of these solutions assume that the characteristic impedance of the transmission line is fixed and known, and therefore terminated accordingly. These solutions result in reduced power available to the transmission line, reduced dynamic range of the signal, and losses and distortion in the signal. Although more pronounced with transmission lines, these problems apply to many electrical circuits.

[0010] For the foregoing reasons, it is desirable to have a method and device for driving and receiving signals on a transmission line that does not exhibit loss of the available transmitter power to drive the line, does not suffer from a reduction in dynamic signal range, and dynamically matches the driving and terminating impedance at the interfaces to the characteristic impedance of the transmission line.

SUMMARY

[0011] The present invention is directed to a method and device for driving a load with active impedance matching that satisfies these needs. The present invention is particularly suitable for providing a method and device for driving and receiving signals on a transmission line that does not exhibit loss of the available transmitter power to drive the line, does not suffer from a reduction in dynamic signal range, and dynamically matches the transmission line interface driving and terminating impedance to the characteristic impedance of the transmission line.

[0012] In a voltage driver version of the present invention, a means is provided for sensing the current provided to a load by a voltage source, and the magnitude of the voltage source is automatically adjusted by negatively feeding back a voltage to an input that represents a scaled value of the sensed current multiplied by an impedance that matches the load impedance. The result is a voltage source having an effective internal impedance that matches the load impedance, but yet maintains full dynamic signal range without a loss of transmitted power to the load.

[0013] In a current driver version of the present invention, a means is provided for sensing the voltage provided to a load by a current source, and the magnitude of the current source is automatically adjusted by negatively feeding back a current to an input that represents a scaled value of the sensed voltage divided by an impedance that matches the load impedance. The result is a current source having an effective internal impedance that matches the load impedance, but yet maintains full dynamic signal range without a loss of transmitted power to the load.

[0014] Although the present method and device is applicable to many electrical circuits, its application is particularly suitable to transmission lines.

[0015] A device having features of the present invention is a device with active impedance matching for driving a load that comprises a voltage driver having an output connected to a load, means for detecting an output current from the voltage driver to the load, means for scaling the detected output current by a scaling value, and means for subtracting a value representing the scaled detected output current from an input signal of the voltage driver. The means for scaling the detected output current may be a multiplier having an input comprising the detected output current and another input comprising the scaling value, an output of the multiplier representing the scaled output current. The device of claim 2, wherein the scaling value is a value representing a load impedance to be matched. The means for scaling the detected output current may be an amplifier having an input comprising the detected output current and a gain equal to the scaling value, an output of the amplifier representing the scaled output current. The means for detecting an output current may be a transformer having a primary winding in series with the output current. The means for detecting an output current may be a resistor in series with the output current and an amplifier with inputs connected to terminals of the resistor. The means for subtracting may be a summing junction of an operational amplifier. The load may be a transmission line. The scaling value may be a characteristic impedance of the transmission line. The means for scaling and the means for subtracting may comprise a digital signal processor.

[0016] In an alternative embodiment of the present invention, a device with active impedance matching for driving a load comprises a current driver having an output connected to a load, means for detecting an output voltage from the current driver to the load, means for scaling the detected output voltage by a scaling value, and means for subtracting a value representing the scaled detected output voltage from an input signal of the current driver. The means for scaling the detected output voltage may be a multiplier having an input comprising the detected output voltage and another input comprising the scaling value, an output of the multiplier representing the scaled output voltage. The scaling value may be a value representing a load impedance to be matched. The means for scaling the detected output voltage may be an amplifier having an input comprising the detected output voltage and a gain equal to the scaling value, an output of the amplifier representing the scaled output voltage. The means for detecting an output voltage may be an amplifier with inputs connected to the outputs of the current driver. The means for detecting an output voltage may be a transformer with primary terminals connected to the outputs of the current driver. The means for subtracting may be a summing junction of an operational amplifier. The load may be a transmission line. The scaling value may be a characteristic impedance of the transmission line. The means for scaling and the means for subtracting may comprise a digital signal processor.

[0017] In another alternative embodiment of the present invention, a method for driving a load with active impedance matching, comprises connecting an output of a voltage driver to a load, detecting an output current from the voltage driver to the load, scaling the detected output current by a scaling value, and subtracting a value representing the scaled detected output current from an input signal of the voltage driver. Scaling the detected output current may comprise multiplying the detected output current by the scaling value, an output of the multiplication representing the scaled output current. The scaling value may be a value representing a load impedance to be matched. The detected output current may comprise amplifying the detected output current by the scaling value for obtaining a value representing the scaled output current. Detecting an output current may comprise connecting a primary winding of a transformer in series with the output current. Detecting an output current may comprise connecting a resistor in series with the output current and connecting inputs of an amplifier to terminals of the resistor. Subtracting may comprise summing currents into a summing junction of an operational amplifier. The load may be a transmission line. The scaling value may be a characteristic impedance of the transmission line. Scaling and subtracting may comprise processing instructions of a digital signal processor.

[0018] In another alternative embodiment of the present invention, a method for driving a load with active impedance matching comprises connecting an output of a current driver to a load, detecting an output voltage from the current driver to the load, scaling the detected output voltage by a scaling value, and subtracting a value representing the scaled detected output voltage from an input signal of the current driver. Scaling the detected output voltage may comprise multiplying the detected output voltage by the scaling value, an output of the multiplication representing the scaled output voltage. The scaling value may be a value representing a load impedance to be matched. Scaling the detected output voltage may comprise amplifying the detected output voltage by the scaling value for obtaining a value representing the scaled output voltage. Detecting an output voltage may comprise connecting inputs of an amplifier to outputs of the current driver. Detecting an output voltage may comprise connecting a primary winding of a transformer to the outputs of the current driver. Subtracting may comprise summing currents into a summing junction of an operational amplifier. The load may be a transmission line. The scaling value may be a characteristic impedance of the transmission line. Scaling and subtracting may comprise processing instructions of a digital signal processor.

[0019] In another alternative embodiment of the present invention, a method for driving a load with active impedance matching comprises connecting an output of a voltage driver to a load, detecting an output current value from the voltage driver to the load, connecting the detected output current to an analog-to-digital converter, converting the detected output current value to a digital representation by the analog-to-digital converter, connecting the digital representation of the output current at an output of the analog-to-digital converter to an input of a digital signal processor, connecting a digital representation of an input signal to another input of the digital signal processor, executing a program in the digital signal processor, providing an digital representation output from the digital signal processor to a digital-to-analog converter, and connecting an output of the digital-to-analog converter to an input of the voltage driver. The method may further comprise interposing an anti-aliasing low-pass filter between the detected current output and the analog-to-digital converter. The method may further comprise interposing an interpolation low-pass filter between the output of the digital-to-analog converter and the input of the voltage driver. The step of connecting a digital representation of an input signal may comprise connecting an input signal to another input of the voltage driver. The step of executing a program in the digital signal processor may further comprise executing an initialization routine, reading an input voltage value, associating a time value with the input voltage value, adjusting the time value with a time domain filter delay, reading an output current value from the analog-to-digital converter, applying the output current value to the time domain filter, subtracting the filtered output current value from the adjusted input voltage value, outputting the result of the subtraction to a digital-to-analog converter, repeating steps b. through h. if the program is not terminated, and ending the process if the program is terminated.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] These and other features, aspects, and advantages of the present invention will become understood with regard to the following description, appended claims, and accompanying drawings where:

[0021]FIG. 1A shows a half duplex configuration of a transmitter/receiver circuit;

[0022]FIG. 1B shows a full duplex configuration of a transmitter/receiver circuit;

[0023]FIG. 2A shows a Thevenin equivalent of a voltage transmitter circuit;

[0024]FIG. 2B shows an equivalent circuit of the circuit shown in FIG. 2A;

[0025]FIG. 3A shows a Norton equivalent of a current transmitter circuit;

[0026]FIG. 3B shows an equivalent circuit of the circuit shown in FIG. 3A;

[0027]FIG. 4 shows a block diagram of a voltage transmitter circuit;

[0028]FIG. 5 shows a block diagram of a current transmitter circuit;

[0029]FIG. 6A shows a circuit diagram of an a voltage transmitter using an inductive sensor;

[0030]FIG. 6B shows a circuit diagram of a voltage transmitter using a resistive sensor;

[0031]FIG. 7 shows a circuit diagram of a DSP implementation of a voltage transmitter;

[0032]FIG. 8A shows a flow diagram of a program executed in the DSP of FIG. 7; and

[0033]FIG. 8B shows a flow diagram of an initialization routine.

DETAILED DESCRIPTION

[0034] Turning now to FIG. 1A, FIG. 1A shows a half duplex configuration 10 of transmitter/receiver circuit connected to a transmission line 140. This configuration is well known to skilled practitioners in the relevant art. In the half duplex configuration of FIG 1A, a transmit input signal 112 is connected to an input of a differential transmitter 102. An output of the transmitter is connected to a switch 106. A receive output signal 114 is provided by an output of a differential receiver 104. An input of the receiver 104 is connected to the switch 106. A transmission line signal 108, 110 is connected to the switch 106 such that the when the switch 106 is in position A, the transmission line signal 108, 110 is connected to the output of the transmitter 102, for transmitting a transmission line signal 108, 110. Alternatively, when the switch is in position B, the transmission line signal 108, 110 is connected to the input of the receiver 104, for receiving a transmission line signal 108, 110. Although the switch 106 is depicted as an electromechanical device, skilled practitioners would recognize that a semiconductor device would normally provide this function for half duplex operation. In half duplex operation, a transmission line 140 is transmitting signals in one direction at a time.

[0035] Turning now to FIG. 1B, FIG. 1B shows a full duplex configuration 12 of transmitter/receiver circuit connected to a transmission line 140. This configuration is also well known to skilled practitioners in the relevant art. In the half duplex configuration of FIG. 1B, a transmit input signal 112 is connected to an input of a differential transmitter 102. An output of the transmitter is connected to a transmission line signal 108, 110 through one port 128 of a hybrid coil or four-to-two wire converter 126. A receive output signal 114 is provided by an output of a differential receiver 104. An input of the receiver 104 is connected the transmission line signal 108, 110 through another port 130 of the hybrid coil or four-to-two wire converter 126. In this full duplex configuration 12, the transmission line 140 may transmit signals in both directions simultaneously, the directional coupling and line termination being performed by the hybrid coil or four-to-two wire converter 126.

[0036] The subsequent descriptions of embodiments of the present invention pertain to the transmitter 102 of FIG. 1A and FIG. 1B. Skilled practitioners will recognize that embodiments of the present invention may be used with a receiver 104 in either a half duplex configuration of FIG. 1A or the full duplex configuration of FIG. 1B, eliminating the need for the switch 106 or the hybrid coil or four-to-two wire converter.

[0037] Turning now to FIG. 2A and FIG. 2B, FIG. 2A shows a Thevenin equivalent circuit 20 of a voltage transmitter circuit and FIG. 2B shows an equivalent circuit 22 of the circuit 20 shown in FIG. 2A. In FIG. 2A, a voltage generator V_(g) 202 represents a Thevenin equivalent open circuit voltage source and an impedance Z_(g) 204 represents a Thevenin equivalent impedance. The circuit has an output voltage V_(o) 208 and an output current I_(o) 206 connected to a load impedance Z_(L) 240. By measuring the output current I_(o) 206 and negatively feeding it back with an appropriate gain required to synthesize the impedance Z_(g) 204, the equivalent circuit 22 shown in FIG. 2B is formed. FIG. 2B comprises a voltage generator 222 having a value of

V _(o) =V _(g) −Z _(g) I _(o)

[0038] that provides the output voltage V_(o) 208 and the output current I_(o) 206 connected to a load impedance Z_(L) 240. Note that V_(o) 208 and I_(o) 206 are the same in FIG. 2A and FIG. 2B. If the value of V_(g) is set to zero (short-circuit) and a current generator of unity value is connected to the outputs of the circuits shown in both FIG. 2A and FIG. 2B, the value of the voltage V_(o)=Z_(g) is the same in both circuits. This example illustrates the principle of operation of one of the embodiments of the present invention. That is, in a voltage transmitter circuit, the source impedance Z_(g) 204 may be matched to a load impedance Z_(L) 240 by measuring the output current from the circuit and negatively feeding back a scaled part of the output current determined by the value of the load impedance Z_(L) 240. In this manner, maximum power transfer may be achieved by setting Z_(g) 204=Z_(L) 240 without power loss in Z_(g) 204.

[0039] Turning now to FIG. 3A and FIG. 3B, FIG. 3A shows a Norton equivalent circuit 30 of a current transmitter circuit and FIG. 3B shows an equivalent circuit 32 of the circuit 30 shown in FIG. 3A. In FIG. 3A, a current generator I_(g) 302 represents a Norton equivalent short circuit current source and an impedance Z_(g) 304 represents a Norton equivalent impedance. The circuit has an output voltage V_(o) 308 and an output current I_(o) 306 connected to a load impedance Z_(L) 340. By measuring the output voltage V_(o) 306 and negatively feeding it back with an appropriate gain required to synthesize the admittance 1/Z_(g) 304, the equivalent circuit 32 shown in FIG. 3B is formed. FIG. 3B comprises a current generator 322 having a value of

I _(o) =I _(g) −V _(o) /Z _(g)

[0040] that provides the output voltage V_(o) 308 and the output current I_(o) 306 connected to the load impedance Z_(L) 340. Note that V_(o) 308 and I_(o) 306 are the same in FIG. 3A and FIG. 3B. If the value of I_(g) is set to zero (open-circuit) and a voltage generator of unity value is connected to the outputs of the circuits shown in both FIG. 3A and FIG. 3B, the value of the current I_(o)=1/Z_(g) is the same in both circuits. This example illustrates the principle of operation of one of the embodiments of the present invention. That is, in a current transmitter circuit, the source impedances Z_(g) 304 may be matched to a load impedance Z_(L) 340 by measuring the output voltage from the circuit and negatively feeding back a scaled part of the output voltage, determined by the value of the load admittances 1/Z_(L) 340. In this manner, maximum power transfer may be achieved by setting Z_(g) 304=Z_(L) 340 without power loss in Z_(g) 304.

[0041] Turning now to FIG. 4, FIG. 4 shows a block diagram of a voltage transmitter circuit 40 connected to a transmission line 440 having a characteristic impedance Z_(o) 442. The block diagram 40 illustrates a use of current feedback 418 from the an output of a voltage driver 410 to synthesize a driver circuit whose Thevenin equivalent is a voltage generator of amplitude 2V_(in) in series with an impedance Z_(o), similar to the circuit shown in FIG. 2A. The input voltage V_(in) 402 is summed with a negative feedback voltage V_(fb) 420 to provide an input voltage of V_(in)−V_(fb) to the voltage driver 410. Since the voltage driver 410 has a voltage gain of two, the output voltage V_(o) 408 of the voltage driver 410 is V_(o)=2V_(in)−2V_(fb). A transformer 412 having a turns ratio of n senses the output current I_(o) 406 and provides the signal I_(o)/n to one input to a multiplier 414. Another input signal to the multiplier is the constant value nZ_(o)/2 516. Therefore, the output signal of the multiplier 414 is V_(fb)=(I_(o)/n)(nZ_(o)/2)=I_(o)Z_(o)/2 420. By substituting this value of V_(fb) 420 into the expression above for the output voltage V_(o) 408, the output voltage

V _(o)=2V _(in) −I _(o) Z _(o)

[0042] This expression for the output voltage V_(o) 408 has the form of the output voltage of FIG. 2A and FIG. 2B, and illustrates how the driving source impedance may be matched to the characteristic impedance of a transmission without the use of power consuming components. Summarizing, the output current 406 is measured, scaled and multiplied by an impedance nZ_(o)/2, resulting in the feedback voltage V_(fb) 420. The feedback voltage V_(fb) 420 is subtracted from the input voltage V_(in) 402 and fed to an input of the voltage driver 410, which has a gain of two. The scaling and multiplication may be accomplished on a current-to-voltage converter, the output driver, or through use of digital filtering techniques in a Digital Signal Processor (DSP). Note that the voltage driver 410 will only generate the voltage seen by the line V_(o), even if the Thevenin equivalent circuit has a voltage generator of twice this value. This method achieves the objective of impedance matching without wasting power or dropping a voltage in an impedance-matching resistor. This method is also adaptable to the use of a DSP to enable more accurate and adaptive matching through digital signal processing techniques. It also allows full duplex communication over the same transmission line.

[0043] Turning now to FIG. 5, FIG. 5 shows a block diagram of a current transmitter circuit 50 connected to a transmission line 540 having a characteristic impedance Z_(o) 542. The block diagram 50 illustrates a use of output voltage feedback V_(o) 508 from an output of a transconductance driver 510 to synthesize a driver circuit 50 whose Norton equivalent circuit is a current generator of amplitude I_(o)=kV_(in) 506 having an internal shunt impedance Z_(g)=Z_(o), the characteristic impedance of the line, and a transconductance of k, similar to FIG. 3A. The input voltage V_(in) 502 is summed with a negative feedback voltage V_(fb) 520 to provide an input voltage of V_(in)−V_(fb) to the transconductance driver 510. Since the transconductance driver 510 has a transconductance of 2k, the output current I_(o) 506 of the transconductance driver 510 is I_(o)=2kV_(in)−2kV_(fb.) An amplifier 512 senses the output voltage V_(o) 508 and provides this signal V_(o) 508 to one input of a multiplier 514. Another input signal to the multiplier is the constant value 1/2kZ_(o) 516. Therefore, the output signal of the multiplier 514 is V_(fb)=V_(o)/2kZ_(o) 520. By substituting this value of V_(fb) 520 into the expression above for the output current I_(o) 506, the output current

I _(o)=2kV _(in) −V _(o) /Z _(o)

[0044] This expression for the output current I_(o) 506 has the form of the output current of FIG. 3A and FIG. 3B, and illustrates how the driving source impedance may be matched to the characteristic impedance of a transmission without the use of power consuming components. Summarizing, the output voltage 508 is measured and multiplied by an admittance 1/2kZ_(o), resulting in the feedback voltage V_(fb) 520. The feedback voltage V_(fb) 520 is subtracted from the input voltage V_(in) 502 and fed to an input of the transconductance driver 510, which has a gain of 2k, where k is the transconductance of the transconductance driver 510. The scaling and multiplication may be accomplished on an amplifier, the output driver, or through use of digital filtering techniques in a Digital Signal Processor (DSP). This method achieves the objective of impedance matching without wasting power or dropping a voltage in an impedance-matching resistor. This method is also adaptable to the use of a DSP to enable more accurate and adaptive matching through digital signal processing techniques. It also allows full duplex communication over the same transmission line.

[0045] Turning now to FIG. 6A, FIG. 6A shows a circuit diagram 60 of a voltage transmitter circuit using an inductive sensor connected to a transmission line 640 having a characteristic impedance Z_(o) 642. The voltage transmitter circuit of FIG. 6A comprises a first operational amplifier 614 and a second operational amplifier 616 having input and outputs connected to a resistor network 618. An input voltage V_(in) 602 is connected to one terminal of an input impedance nZ_(o)/4 603. Another terminal of the input impedance nZ_(o)/4 603 connects a summing junction 620. An output voltage V_(o) 608 is derived between an output terminal of the first amplifier 614 and an output terminal of the second amplifier 616. The voltages at the output terminals of the amplifiers are mirror images of each other. That is, when the output terminal of the first amplifier is at given voltage, the output terminal of the second amplifier is at an equal voltage of opposite polarity. The magnitude of the voltage at the output terminal of each amplifier 614, 616 is V_(o)/2. A terminal of a feedback impedance nZ_(o)/2 604 is connected to the output terminal of the second amplifier 616 and another terminal of the feedback impedance nZ_(o)/2 604 is connected to the summing junction 620. A transformer 612 senses the output current I_(o) 606 and provides a scaled feedback current I_(fb)=I_(o)/n 622 to the summing junction 620. By summing the currents into the summing junction, an expression for the output voltage may be derived

V _(o)=4V _(in) −I _(o) Z _(o)

[0046] This expression for the output voltage V_(o) 608 has the form of the output voltage of FIG. 2A and FIG. 2B, and illustrates how the driving source impedance may be matched to the characteristic impedance of a transmission without the use of power consuming components.

[0047] Turning now to FIG. 6B, FIG. 6B shows a circuit diagram 65 of a voltage transmitter circuit using a resistive sensor connected to a transmission line 690 having a characteristic impedance Z_(o) 692. FIG. 6B is similar to FIG. 6A, except that a current sensing resistor R_(s) 662, a differential amplifier 660 with a gain of A and a resistor nR_(s)/A 674 have replaced the current sensing transformer of FIG. 6A. The inputs of the amplifier 660 are connected to the terminals of the sensing resistor R_(s) 662. The output current I_(o) 656 through the sensing resistor R_(s) 662 creates a voltage that is detected by the amplifier 660. The output of the amplifier 660 is connected to a terminal of the resistor nR_(s)/A 674 and another terminal of the resistor nR_(s)/A 674 is connected to a summing junction 670. The voltage transmitter circuit of FIG. 6B further comprises a first operational amplifier 664 and a second operational amplifier 666 having input and outputs connected to a resistor network 668. An input voltage V_(in) 652 is connected to one terminal of an input impedance nZ_(o)/4 653. Another terminal of the input impedance nZ_(o)/4 653 connects the summing junction 670. An output voltage V_(o) 658 is derived between an output terminal of the first amplifier 664 and an output terminal of the second amplifier 666. The voltages at the output terminals of the amplifiers are mirror images of each other. That is, when the output terminal of the first amplifier is at given voltage, the output terminal of the second amplifier is at an equal voltage of opposite polarity. The magnitude of the voltage at the output terminal of each amplifier 664, 666 is V_(o)/2. A terminal of a feedback impedance nZ_(o)/2 654 is connected to the output terminal of the second amplifier 666 and another terminal of the feedback impedance nZ_(o)/2 654 is connected to the summing junction 670. By summing the currents into the summing junction, an expression for the output voltage may be derived:

V _(o)=4V _(in) −I _(o) Z _(o)

[0048] This expression for the output voltage V_(o) 658 has the form of the output voltage of FIG. 2A and FIG. 2B, and illustrates how the driving source impedance may be matched to the characteristic impedance of a transmission without the use of power consuming components.

[0049] Turning now to FIG. 7, FIG. 7 shows a circuit diagram 70 of a DSP implementation of a voltage transmitter connected to a transmission line 740 having a characteristic impedance Z_(o) 742. The amplifiers 714, 716, resistor network 718, input resistor 703, feedback resistor 704, and transformer 712 are similar to those corresponding elements shown in FIG. 6A. The voltage transmitter circuit of FIG. 7 comprises a first operational amplifier 714 and a second operational amplifier 716 having input and outputs connected to a resistor network 718. An input resistor 703 connects between an output of an interpolation filter 728 and a summing junction 734. A feedback resistor 704 connects between the summing junction 734 and an output of the second amplifier. An output voltage V_(o) 708 is derived between an output terminal of the first amplifier 714 and the output terminal of the second amplifier 716. The voltages at the output terminals of the amplifiers are mirror images of each other. That is, when the output terminal of the first amplifier is at given voltage, the output terminal of the second amplifier is at an equal voltage of opposite polarity. The magnitude of the voltage at the output terminal of each amplifier 714, 716 is V_(o)/2. A transformer 712 senses the output current I_(o) 706 and provides a scaled feedback current I_(o)/n to the summing junction 736 of an I/V converter 730. A feedback resistor 732 connects between an output of the I/V converter 730 and the summing junction 736 of the I/V converter 730. The output of the I/V converter 730 is connected to the input of an anti-aliasing filter 720. An output of the anti-aliasing filter 720 is connected to an input of an analog-to-digital (A/D) converter 722. Outputs from the A/D converter 722 are connected to a DSP 724. Outputs from the DSP 724 are connected to the inputs of a digital-to-analog (D/A) converter 726. An output from the D/A converter 726 is connected to the interpolation filter 728. Normally the DSP generates the signals to be transmitted over the transmission line, functioning as a modem. Alternatively, a digital input voltage V_(in) 702 is connected an input terminal of the DSP. By performing scaling and feedback functions in a DSP 724, intelligence is added to the process that allows sophisticated and adaptive matching of the characteristic impedance Z_(o) of the transmission line. The DSP 724 may send a voltage signal V_(o) to the line, measure the resulting current and calculate a transfer function, such as nV/I. With sufficient over-sampling to avoid excessive phase shift, the line impedance may be matched by multiplying the line current I_(o) by a suitable transfer function and subtracting the result from twice the intended output signal V_(o.) For full duplex operation, the received signal may be obtained by digitally subtracting the transmitted signal from the line voltage V_(o) measured by a receiver. Since the invention requires the use of line impedance models, with a DSP, these models are no longer limited to simple passive network elements.

[0050] Turning now to FIG. 8A, FIG. 8A shows a representative flow diagram 80 of a program executed in the DSP of FIG. 7. The DSP is started 802 whenever it is initially powered on or reset. A first step is an execution of an initialization routine 804. The details of the initialization routine 804 are described in the description of FIG. 8B. The DSP then reads a value representing an input voltage 806, associates a current time value with the input voltage 808, and adjusts the time value for a time domain filter delay 810. Concurrently with these steps, the DSP reads a value representing an output current I_(o)/n from an A/D converter 812, calculates an error from a predicted current and updates time domain filter 813, and applies the output current value to a nZ_(o)/2 time domain filter 814. The DSP then subtracts the filtered output current value from the input voltage value 816, and provides the resultant value to a D/A converter. If the DSP operation is to be terminated 820, the process is ended 822. If not terminated 820, the process beginning with concurrently reading input voltage values 806 and reading output current values 812 is repeated. As an alternative to the initialization routine 804 described in FIG. 8B, the initialization routine may be limited to setting initial parameters of the time domain filter for synthesizing an output impedance of approximate value. Then, referring to FIG. 8A, the DSP would read the output current 812, calculate an error from a predicted current and update the time domain filter 813 with a fraction of the error to improve the matching in a recursive manner. These updated values would then be used to adjust the output voltage 816.

[0051] Turning now to FIG. 8B, FIG. 8B shows a flow diagram 85 of an initialization routine depicted as step 804 in FIG. 8A. If the DSP requires initialization, as described in the description of FIG. 8A, the initialization routine is started 850. A value of the characteristic impedance Z_(o) of the transmission line is set to approximately match the transmission line and this value is applied to a time domain filter 852. For example, an approximate value of 600 ohms is used for telephone lines, 120 ohms for twisted pair, or 50 ohm for coaxial cable. The DSP then initiates a request to a receiver at the opposite end of the transmission line to present a short-circuit for a fixed amount of time 854, simulates a short-circuit output by setting an output voltage to a constant 856, and measures the value of an output current to find values for a short-circuit impedance Z_(is) versus frequency 858. The DSP then initiates a request to the receiver at the opposite end of the transmission line to present an open-circuit for a fixed amount of time 860, simulates an open-circuit output by setting an output voltage to zero and setting Z_(o) to a high value 862, and measures the value of the output current to find values for an open circuit impedance Z_(io) versus frequency 864. The DSP then computes values for the characteristic impedance Z_(o)=(Z_(is) Z_(os))^(½) versus frequency 866 and sets time domain filter parameters to match Z_(o) versus frequency 869. Control is then returned to the main program 870.

[0052] Although the present invention has been described in detail with reference to certain preferred embodiments, it should be apparent that modifications and adaptations to those embodiments may occur to persons skilled in the art without departing from the spirit and scope of the present invention as set forth in the following claims. 

What is claimed is:
 1. A device for active impedance matching, comprising: a. a voltage driver having an output connected to a load; b. a resistor in series with the output current and an amplifier with inputs connected to terminals of the resistor for detecting an output current from the voltage driver to the load; c. means for scaling the detected output current by a scaling value; and d. means for subtracting a value representing the scaled detected output current from an input signal of the voltage driver.
 2. The device of claim 1, wherein the means for scaling and the means for subtracting comprises a digital signal processor.
 3. The device of claim 1, wherein the means for scaling the detected output current is an amplifier having an input comprising the detected output current and a gain equal to the scaling value, an output of the amplifier representing the scaled output current.
 4. A device for active impedance matching, comprising: a. a current driver having an output connected to a load; b. means for detecting an output voltage from the current driver to the load; c. means for scaling the detected output voltage by a scaling value; and d. means for subtracting a value representing the scaled detected output voltage from an input signal of the current driver.
 5. The device of claim 4, wherein the means for scaling the detected output voltage is a multiplier having an input comprising the detected output voltage and another input comprising the scaling value, an output of the multiplier representing the scaled output voltage.
 6. The device of claim 5, wherein the scaling value is a value representing a load impedance to be matched.
 7. The device of claim 4, wherein the means for scaling the detected output voltage is an amplifier having an input comprising the detected output voltage and a gain equal to the scaling value, an output of the amplifier representing the scaled output voltage.
 8. The device of claim 4, wherein the means for detecting an output voltage is a differential amplifier with inputs connected to the outputs of the current driver.
 9. The device of claim 4, wherein the means for subtracting is a summing junction of an operational amplifier.
 10. The device of claim 4, wherein the load is a transmission line.
 11. The device of claim 10, wherein the scaling value is a characteristic impedance of the transmission line.
 12. The device of claim 4, wherein the means for scaling and the means for subtracting comprises a digital signal processor.
 13. A method of active impedance matching, comprising: a. connecting an output voltage driver to a load; b. detecting an output current from the voltage driver to the load; c. scaling the detected output current by a scaling value by processing scaling instructions using a digital signal processor; and d. subtracting a value representing the scaled detected output current from an input signal of the voltage driver by processing subtraction instructions using a digital signal processor.
 14. A method for active impedance matching, comprising: a. connecting an output of a current driver to a load; b. detecting an output voltage from the current driver to the load; c. scaling the detected output voltage by a scaling value; and d. subtracting a value representing the scaled detected output voltage from an input signal of the current driver.
 15. The method of claim 14, wherein scaling the detected output voltage comprises multiplying the detected output voltage by the scaling value, an output of the multiplication representing the scaled output voltage.
 16. The method of claim 15, wherein the scaling value is a value representing a load impedance to be matched.
 17. The method of claim 14, wherein scaling the detected output voltage comprises amplifying the detected output voltage by the scaling value for obtaining a value representing the scaled output voltage.
 18. The method of claim 14, wherein detecting an output voltage comprises connecting inputs of a differential amplifier to outputs of the current driver.
 19. The method of claim 14, wherein subtracting comprises summing currents into a summing junction of an operational amplifier.
 20. The method of claim 14, wherein the load is a transmission line.
 21. The method of claim 20, wherein the scaling value is a characteristic impedance of the transmission line.
 22. The method of claim 14, wherein scaling and subtracting comprises processing instructions of a digital signal processor.
 23. A method for active impedance matching, comprising: a. connecting an output of a voltage driver to a load; b. detecting an output current value from the voltage driver to the load; c. connecting the detected output current to an analog-to-digital converter; d. converting the detected output current value to a digital representation by the analog-to-digital converter; e. connecting the digital representation of the output current at an output of the analog-to-digital converter to an input of a digital signal processor; f. connecting a digital representation of an input signal to another input of the digital signal processor; g. executing a program in the digital signal processor; h. providing a digital representation output from the digital signal processor to a digital-to-analog converter; and i. connecting an output of the digital-to-analog converter to an input of the voltage driver.
 24. The method of claim 23, further comprising interposing an anti-aliasing low-pass filter between the detected current output and the analog-to-digital converter.
 25. The method of claim 23, further comprising interposing an interpolation low-pass filter between the output of the digital-to-analog converter and the input of the voltage driver.
 26. The method of claim 23, wherein the step of connecting a digital representation of an input signal comprises connecting the input signal to another input of the voltage driver.
 27. The method of claim 23, wherein the step of executing a program in the digital signal processor further comprises: a. executing an initialization routine; b. reading an input voltage value; c. associating a time value with the input voltage value; d. adjusting the time value with a time domain filter delay; e. reading an output current value from the analog-to-digital converter; f. applying the output current value to the time domain filter; g. subtracting the filtered output current value from the adjusted input voltage value; h. outputting the result of the subtraction to a digital-to-analog converter; i. repeating steps b. through h. if the program is not terminated; and j. ending the process if the program is terminated. 